cgroup: pass around cgroup_subsys_state instead of cgroup in subsystem methods

cgroup is currently in the process of transitioning to using struct
cgroup_subsys_state * as the primary handle instead of struct cgroup *
in subsystem implementations for the following reasons.

* With unified hierarchy, subsystems will be dynamically bound and
  unbound from cgroups and thus css's (cgroup_subsys_state) may be
  created and destroyed dynamically over the lifetime of a cgroup,
  which is different from the current state where all css's are
  allocated and destroyed together with the associated cgroup.  This
  in turn means that cgroup_css() should be synchronized and may
  return NULL, making it more cumbersome to use.

* Differing levels of per-subsystem granularity in the unified
  hierarchy means that the task and descendant iterators should behave
  differently depending on the specific subsystem the iteration is
  being performed for.

* In majority of the cases, subsystems only care about its part in the
  cgroup hierarchy - ie. the hierarchy of css's.  Subsystem methods
  often obtain the matching css pointer from the cgroup and don't
  bother with the cgroup pointer itself.  Passing around css fits
  much better.

This patch converts all cgroup_subsys methods to take @css instead of
@cgroup.  The conversions are mostly straight-forward.  A few
noteworthy changes are

* ->css_alloc() now takes css of the parent cgroup rather than the
  pointer to the new cgroup as the css for the new cgroup doesn't
  exist yet.  Knowing the parent css is enough for all the existing
  subsystems.

* In kernel/cgroup.c::offline_css(), unnecessary open coded css
  dereference is replaced with local variable access.

This patch shouldn't cause any behavior differences.

v2: Unnecessary explicit cgrp->subsys[] deref in css_online() replaced
    with local variable @css as suggested by Li Zefan.

    Rebased on top of new for-3.12 which includes for-3.11-fixes so
    that ->css_free() invocation added by da0a12caff ("cgroup: fix a
    leak when percpu_ref_init() fails") is converted too.  Suggested
    by Li Zefan.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Aristeu Rozanski <aris@redhat.com>
Acked-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 25 insertions, 23 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7a10742b389a..622b7efc5ade 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7094,16 +7094,17 @@ static inline struct task_group *cgroup_tg(struct cgroup *cgrp)
         return css_tg(cgroup_css(cgrp, cpu_cgroup_subsys_id));
 }
 
-static struct cgroup_subsys_state *cpu_cgroup_css_alloc(struct cgroup *cgrp)
+static struct cgroup_subsys_state *
+cpu_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
 {
-        struct task_group *tg, *parent;
+        struct task_group *parent = css_tg(parent_css);
+        struct task_group *tg;
 
-        if (!cgrp->parent) {
+        if (!parent) {
                 /* This is early initialization for the top cgroup */
                 return &root_task_group.css;
         }
 
-        parent = cgroup_tg(cgrp->parent);
         tg = sched_create_group(parent);
         if (IS_ERR(tg))
                 return ERR_PTR(-ENOMEM);
@@ -7111,38 +7112,38 @@ static struct cgroup_subsys_state *cpu_cgroup_css_alloc(struct cgroup *cgrp)
         return &tg->css;
 }
 
-static int cpu_cgroup_css_online(struct cgroup *cgrp)
+static int cpu_cgroup_css_online(struct cgroup_subsys_state *css)
 {
-        struct task_group *tg = cgroup_tg(cgrp);
-        struct task_group *parent = css_tg(css_parent(&tg->css));
+        struct task_group *tg = css_tg(css);
+        struct task_group *parent = css_tg(css_parent(css));
 
         if (parent)
                 sched_online_group(tg, parent);
         return 0;
 }
 
-static void cpu_cgroup_css_free(struct cgroup *cgrp)
+static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
 {
-        struct task_group *tg = cgroup_tg(cgrp);
+        struct task_group *tg = css_tg(css);
 
         sched_destroy_group(tg);
 }
 
-static void cpu_cgroup_css_offline(struct cgroup *cgrp)
+static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
 {
-        struct task_group *tg = cgroup_tg(cgrp);
+        struct task_group *tg = css_tg(css);
 
         sched_offline_group(tg);
 }
 
-static int cpu_cgroup_can_attach(struct cgroup *cgrp,
+static int cpu_cgroup_can_attach(struct cgroup_subsys_state *css,
                                  struct cgroup_taskset *tset)
 {
         struct task_struct *task;
 
-        cgroup_taskset_for_each(task, cgrp, tset) {
+        cgroup_taskset_for_each(task, css->cgroup, tset) {
 #ifdef CONFIG_RT_GROUP_SCHED
-                if (!sched_rt_can_attach(cgroup_tg(cgrp), task))
+                if (!sched_rt_can_attach(css_tg(css), task))
                         return -EINVAL;
 #else
                 /* We don't support RT-tasks being in separate groups */
@@ -7153,18 +7154,18 @@ static int cpu_cgroup_can_attach(struct cgroup *cgrp,
         return 0;
 }
 
-static void cpu_cgroup_attach(struct cgroup *cgrp,
+static void cpu_cgroup_attach(struct cgroup_subsys_state *css,
                               struct cgroup_taskset *tset)
 {
         struct task_struct *task;
 
-        cgroup_taskset_for_each(task, cgrp, tset)
+        cgroup_taskset_for_each(task, css->cgroup, tset)
                 sched_move_task(task);
 }
 
-static void
-cpu_cgroup_exit(struct cgroup *cgrp, struct cgroup *old_cgrp,
-                struct task_struct *task)
+static void cpu_cgroup_exit(struct cgroup_subsys_state *css,
+                            struct cgroup_subsys_state *old_css,
+                            struct task_struct *task)
 {
         /*
          * cgroup_exit() is called in the copy_process() failure path.
diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
index f6926a149a71..1b784d9b3630 100644
--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -62,11 +62,12 @@ static struct cpuacct root_cpuacct = {
 };
 
 /* create a new cpu accounting group */
-static struct cgroup_subsys_state *cpuacct_css_alloc(struct cgroup *cgrp)
+static struct cgroup_subsys_state *
+cpuacct_css_alloc(struct cgroup_subsys_state *parent_css)
 {
         struct cpuacct *ca;
 
-        if (!cgrp->parent)
+        if (!parent_css)
                 return &root_cpuacct.css;
 
         ca = kzalloc(sizeof(*ca), GFP_KERNEL);
@@ -92,9 +93,9 @@ out:
 }
 
 /* destroy an existing cpu accounting group */
-static void cpuacct_css_free(struct cgroup *cgrp)
+static void cpuacct_css_free(struct cgroup_subsys_state *css)
 {
-        struct cpuacct *ca = cgroup_ca(cgrp);
+        struct cpuacct *ca = css_ca(css);
 
         free_percpu(ca->cpustat);
         free_percpu(ca->cpuusage);